Various forms of plant protectors are shown in numerous patents, some of which resemble the currently popular "Hot Cap", as in U.S. Pat. No. 995,657, granted in 1911 in which a paper or wood veneer frusto-conical protector is spaced slightly above the ground and its open apex permits flow of air about the plant while protecting it from heavy winds and rain, as well as from extreme cold or heat. However, the opaque nature of this cover would not allow adequate sunlight to play on the plant. Additionally, the invention is not generally adaptable to protection for an entire bed. A somewhat similar "Ventilated Plant Shelter" is shown in U.S. Pat. No. 3,360,884, in which a ventilating tube extends from a closed upper end of a bag which is anchored to the ground for enclosing a plant. It appears questionable whether or not the plant would receive adequate ventilation and might be overheated. A "cloth house" shown in U.S. Pat. No. 2,051,643, is intended to cover many acres with a net like top and sides secured to a base board. Other than partially shading the plants, a structure of this type would have little if any effect in retaining cool air about the plants while permitting warm air to escape freely from the top of the structure. Another patent of passing interest is U.S. Pat. No. 3,812,616, in which a transparent, portable greenhouse has side walls which may be rolled up for tending the plants. However, permitting cool air about the plants with hot air rising from the bed area and escaping from the greenhouse is not provided.
Two patents granted to Drake of DeLand, Florida, early this century, U.S. Pat. No. 898,341, and U.S. Pat. No. 932,971, show structures for protecting cool weather crops and are of particular interest. The older patent shows a structure with one side wall and one end wall, the other side and end being open. Slats seated on the top edge of the end wall and atop bars provide some shelter from the sun, and a removable burlap cover can be extended across the slats. The slats are so far apart that protection from very intense sun would be ineffective, and a lack of a complete wall about the area would not retain cool air about the plants and would provide but limited wind protection. In the later patent, U.S. Pat. No. 932,971, the prior structure is modified by a complete wall about the area, but no ventilation is provided along the bottom of the wall. In both of the patents when a cover is applied over the bed, free discharge of hot air is limited if not prevented, but the cover is primarily a frost preventative.
It should further be noted that both cold frames and hot frames are intended for retaining seedlings and young plants adequately warm in cool weather, and while they generally have tops which may be opened for ventilation, there is generally no provision for retaining cool air around the plants while permitting warm air to rise and escape, but rather the intent and structure is the opposite of such function.